Abstract: A method and apparatus are described for diagnosing the condition of oil-wetted parts subject to wear, wherein a specimen of the used oil from the part is extracted, and a plurality of samples are separated according to the different mobility rates of its particles through a liquid medium during the centrifuging of the specimen through the liquid medium for a predetermined period of time. The samples are then analyzed to determine their compositions, thereby providing an indication of the identities of the wearing parts and their rates-of-wear. In one described embodiment, the specimen is centrifuged for a sufficiently long predetermined time to stratify the particles according to size and density; and in a second described embodiment, the specimen is centrifuged for a plurality of short predetermined time periods during each of which a sample of the particles passing the complete length of the liquid medium is removed and analyzed.